How long does it take to learn parkour?

I'm by no means a parkour expert, but I think the answer will depend greatly on your level of fitness, muscle memory, ability to stretch, and whether you've been involved in similar activities or sports that will help you ease into the demands of parkour.

Then there are other factors which you may not even have complete control over, such as your balance or height.

Progress takes time though, and the most important thing to get good at something as demanding as parkour is that you are committed, patient, and play it safe. In many ways, the answer is simply "a lifetime", as you will get incrementally better day by day, and then one day you'll realize you are considered an 'expert'.

Practice flips and similar techniques in the gym, and even when you're outdoors use a mat and have someone spotting you.